The invention relates to an echo suppressor and a non-linear processor for an echo canceller in a 4-wire data transmission network.
In bidirectional data transmission networks, such as telephone networks, echo occurs on end-to-end connections, as the talking party""s voice is reflected from certain network elements. The echo is disturbing if there is delay on the end-to-end connection. The delay is usually either propagation delay or delay caused by digital signal processing.
Echo is divided into two categories: acoustic echo between the earpiece and microphone of a telephone, and electric echo caused in the transmission systems of the transmission and reception directions of a connection.
One of the main reasons behind electric echo is hybrid circuits (2-wire-4-wire converters) in terminal exchanges or remote subscriber stages of a fixed network. The subscriber lines of a fixed network are usually 2-wire lines for economic reasons, whereas connections between exchanges are 4-wire connections.
In this application, the end of a transmission connection to which the talking party""s own voice returns as an echo is referred to as the far end, whereas the end of the connection from which the echo is reflected back is referred to as the near end.
An echo canceller or an echo suppressor has conventionally been used to obviate problems caused by echo. An echo canceller is a device for processing a signal, such as a speech signal. It estimates the echo and reduces the echo by subtracting the echo estimate from a signal returning from the echo path (from the near end). In echo estimation, the impulse response of the echo path is usually modelled by an adaptive filter. In addition, a non-linear processor (NLP) is often used in echo cancellers for removing residual echo resulting from adaptive filtering.
An echo suppressor is based on comparison between the power levels of a signal supplied to the echo path and a signal returning therefrom. If the ratio of the power level of the signal returning from the echo path to the power level of the signal supplied to the echo path is lower than a pre-determined ratio, the transmission connection returning from the echo path will be disconnected, whereby the echo is not allowed to pass through. Otherwise it is interpreted that either near-end speech or double talk (simultaneous near-end and far-end speech) is in question, in which case the connection naturally cannot be disconnected. The non-linear processor (NLP) or center clipper used for eliminating residual echo in echo cancellers is also a certain kind of echo suppressor.
At present, mainly echo cancellers are used for eliminating echo, since echo suppressors cause the following problems. As the reference ratio for the signals of the near and far end must be selected according to the worst echo situation (usually xe2x88x926 dB), low-level near-end speech does not pass through an echo suppressor during double talk. Even if the average speech levels of the near and far end were equal, near-end speech is clipped occasionally during double talk, depending on the ratio between the signal levels. Another problem is echo during double talk. During double talk, near-end speech passes through the echo suppressor, and so does echo of far-end speech when summed to the near-end speech. The echo of double talk can be reduced by attenuating the near-end signal and possibly even the far-end signal in the echo suppressor during double talk. However, the attenuation cannot be too high, since it has a disturbing xe2x80x9cpumpingxe2x80x9d effect on the strength of the speech.
Although echo cancellers are technically better than echo suppressors, there are situations in which it is justified to use an echo suppressor. In practice, the adaptive filter of an echo canceller should be implemented digitally, which may be too expensive in a purely analogue data transmission system, particularly in terminals. Even in digital data transmission systems, the adaptive filter requires either a specific ASIC or a signal processor, the prices and current consumption of which may be too high for portable terminals, for example.
It is justifiable to use an echo suppressor in a data transmission network, i.e. not in a terminal, if the adaptive filter of an echo canceller is not sufficiently efficient. An adaptive filter removes echo poorly if the echo path is non-linear, i.e. if the non-linear distortion ratio of returning echo is poor. Non-linearity is caused, for instance, by speech coding of low transmission rate. Speech coding can be used on both fixed and wireless transmission connections.
The object of the present invention is to improve the double talk dynamics of an echo suppressor.
The invention relates to an echo suppressor for eliminating acoustic echo, said echo suppressor comprising
means for determining properties of far-end and near-end signals,
means for enabling or disabling transmission of a near-end signal to the far end, depending on said properties of the far end and near end. The echo suppressor is characterized in that it further comprises
means for treating the spectrum of a far-end signal, before said properties are determined, in a manner which models the effect of the amplitude response of the transfer function of acoustic echo.
The invention also relates to a non-linear processor for an echo canceller, comprising
means for determining signal power levels for the far end and near end,
means for activating or deactivating the non-linear processor according to the properties of the far-end and near-end signals. The non-linear processor is characterized in that it further comprises
means for treating the spectrum of a far-end signal, before said properties are determined, in a manner which models the effect of the amplitude response of the transfer function of acoustic echo on residual echo, and that
said near-end signal power level is the power level of the residual echo of the echo canceller.
One of the properties of acoustic echo is that the amplitude response of its transfer function (the frequency response difference between the echo supplied to the echo path and the echo returning therefrom) is very uneven. Particularly the amplitude response of the acoustic connection between the earpiece and microphone of a terminal is extremely uneven in the frequency domain: the amplitude response typically comprises a peak in the frequency range of about 1.5 to 3.0 kHz. The echo return loss ERL is thus clearly a function of frequency, i.e. ERL on the echo path is considerably lower at the peak than for instance at lower frequencies.
The invention utilizes this property in the control of an echo suppressor. As stated above, the control of an echo suppressor is based on the comparison between certain properties, such as power levels, of the near-end and far-end signals. According to the invention, the spectrum of the far-end signal is treated before the signal power level is determined in a manner which models the effect of the amplitude response of the transfer function of acoustic echo. The spectrum may be treated, for example, with a weighted filter whose amplitude response in the frequency domain is optimized to substantially correspond to the amplitude response of the transfer function of acoustic echo. In other words, the weighted filter aims at modelling echo return loss ERL as a function of frequency. If the near-end signal power level is lower than the frequency weighted far-end signal power level, the near-end signal is interpreted as acoustic echo, and the signal returning from the near end is not allowed to pass through the echo suppressor. If the near-end signal power level is higher than the frequency weighted far-end signal power level, the near-end signal is interpreted either as near-end speech or as double talk, and the near-end signal is allowed to pass through the echo suppressor.
The invention improves the double talk dynamics of an echo suppressor by the following mechanism. In a conventional echo suppressor based on unweighted power level comparison, high-energy vowels from the far end clip low-energy consonants, and partly also low-level vowels, from the near end with a high probability during double talk. The weighted filter of the invention, which is typically of high-pass or band-pass type, reduces the energy of the high-energy vowels of far-end speech in relation to the low-energy consonants. This is because the energy of vowels lies mainly in frequencies below 1 kHz, whereas the energy of consonants is distributed fairly evenly over the entire speech-frequency range. The energy of vowels of a far-end signal treated with a weighted filter is thus lower than in the known echo suppressors. Vowels of near-end speech are thus not clipped, and in addition, consonants are less likely to be clipped during double talk than in the known echo suppressors. Only high-energy consonants of the far end can clip low-energy consonants of near-end speech. Since consonants are short as compared with vowels, any clipping times of consonants of near-end speech are short and hardly deteriorate the near-end talk noticeably.